1. Field of the Invention
The present invention relates to a connecting rod and to a method of producing a connecting rod and, more particularly, to a connecting rod with an elliptical opening that eliminates the need for bearings.
2. Background Information
Typically, internal combustion engines include a connecting rod for connecting pistons to a crankshaft. The function of the connecting rod is to transfer power from the pistons to the crankshaft. The connecting rod must act as a unitary piece to transfer dynamic forces with better life. Connecting rods for coupling pistons and crankshafts are designed with a small ring-shaped end and a large ring-shaped end joined together by a rigid member that is connected to the outer periphery of each ring portion. The small ring-shaped end is designed as a bushing or as a press-fit connection for receiving a bearing for rotationally receiving a piston pin, and the large end is designed for rotationally receiving the crankshaft.
Most connecting rods may be broadly categorized in two types. The first more complex type is a split ring connecting rod, or so-called “cut” or “cracked” connecting rod in which the large connecting rod opening that surrounds the crankshaft is cut or cracked to open it. As a rule, the small connecting rod opening does not need to be opened since it is connected to the piston by a straight bolt. In such connecting rods, the large end surrounding the crankshaft is formed as two C-shaped halves, each containing an inner bearing surface and either bolted together across the joint, or split, formed by the ends of the C-shaped section in position about the crankshaft. The second type is a one-piece connecting rod in which the ring section forming a bearing around the crankpin is continuous, without bolted joints. The simplicity of the one-piece connecting rod is offset by more demanding considerations in crankshaft design. One piece connecting rods require the crankshaft to be either assembled around the connecting rods or to be designed in such a way as to allow the engine to be assembled by passing the large circular shaped end of the connecting rods over an end of the crankshaft and along the crankshaft onto the crankpin.
To allow the crankshaft to rotate properly within the larger opening of the connecting rod, bearings are inserted. Depending upon the load applied to the bearing, connecting rod openings are made with a variety of bearing shells providing a friction surface. In particular, supporting shell materials used in bearing shells are as a rule made of C 10 steel according to DIN 17210 or SAE 1010. Depending upon the particular design and application, the bearing shells may be cold hardened. The actual bearing surface layer, which may, for example, be white metal, leaded bronze, light metal, spatter coatings or the like depending upon the expected bearing load, may be applied to the supporting shell material. The bearing shells may be three-component, two-component or solid single component bearing shells. The shells are assembled to the connecting rod opening with an initial stress so that the bearing shells have a satisfactory firm seat upon assembly.
Operationally reliable connecting rods require a wear-resistant design and construction in order to transmit the bearing forces reliably and at permissible operating temperatures. Wear resistance is always provided when the sliding surfaces are separated from each other by a lubricating film such as an oil film that is capable of bearing a load. As a result, the bearings are designed to provide just enough clearance between the inner surface of the bearings and the outer surface of the crankshaft to allow for a hydrodynamic oil film. Thus, friction is minimized resulting in relatively low heat generation. However, when running in a failure mode, for example when an engine runs out of oil, there is a loss of lubrication between the bearing and the crankshaft, and heat rapidly builds up in the bearing. The loss of oil film also results in increased friction between the rotating surfaces of the crankshaft and bearing which in turn increases the load on the engine. This increased load also further increases friction. As a result there is an exponential increase in the amount of heat generated between these two rotating surfaces which occurs in a very short period of time. The temperature of the bearing thus increases at an extremely rapid rate, and, if the engine is not stopped, will reach the yield temperature of the components resulting in self-destruction of the bearing and massive damage to the interior of the engine.
While the engine is running, a tremendous amount of axial tension is placed on the connecting rod. If a bearing is designed with inadequate clearance, the axial tension deforms the bearing during operation squeezing out the oil film and constricting the bearing around the crankshaft. As described above, friction and heat generation increase, as well as the risk of premature wear and mechanical failure. To prevent this, the inner surfaces of bearings are designed to be elliptical in cross section, rather than circular. This allows the bearings to flex under the axial tension to become circular during operation, allowing the proper clearance for a hydrodynamic oil film. In conventional connecting rods, it is the inner surface of the bearings that are machined with this elliptical surface. To facilitate easy assembly, the connecting rod opening and the outside surface of the bearing are machined to be circular.
Conventional connecting rods are usually manufactured in a two-step process; the caps are sawed from the rod and the caps and rod ends are machined, and then fastened together and the bearing hole bored and honed. The fasteners are removed and the bearing shells are inserted and the rod assembly is fastened to the crankshaft. If the fastening is not adequate at either of these stations the chances for a non-uniform oil thickness or the chances of an improperly shaped hole is highly probable. These conditions alone can lead to a scored crank, spun bearing or even worse—a blown engine. Internal engine repairs and replacements are very expensive, both in monetary value and in customer satisfaction value.
While the function that bearings provide is highly desirable, adding bearings to each connecting rod adds extra weight and cost to the overall engine. Carmakers are increasingly looking to increase the efficiency of vehicles by reducing the overall weight. This is especially true within racing circuits, such as NASCAR, where small advantages in efficiency can make the difference between winning and losing a race. As a result, the additional weight added by connecting rod bearings is undesirable.
In the past, various types of connecting rods have been used without bearings. An example of a connecting rod without bearings is shown in U.S. Pat. No. 6,329,022 B1. This patent discloses a method for plasma-coating the surface of the larger connecting rod opening with a bearing material. This provides a high strength-bearing layer directly to the opening surface that is resistant to both wear and high temperatures. In addition, it asserts that with this strength-bearing layer, bearings are no longer needed allowing for a thicker and stronger connecting rod which can withstand higher loads. While the '022 patent provides a surface that is resistant to the wear and high temperatures caused by higher loads, by building up the connecting rod to help it withstand the higher loads. Essentially, additional material and weight is added onto the connecting rod to make up for the eliminated bearings. As a result, there is no reduction in weight and, therefore, no gain in overall efficiency. In addition, the connecting rod in the '022 patent is still susceptible to increased axial tension deforming the connecting rod opening resulting in non-uniform clearance between the crankshaft and the opening surface. For the reasons described above, this destroys the hydrodynamic oil film increasing the risk of premature wear and mechanical failure. Therefore, there is a long felt need for a connecting rod that can effectively connect to a crankshaft without bearings.